How far is the upper bound of photovoltaic conversion efficiency from the paramount efficiency of work extraction from statistically deformed thermal radiation energy?

Abstract

The paper theorizes on the ultimate potential of the photovoltaic way of extracting work from thermal radiation. Two materials very well suited for photovoltaic energy conversion are considered: the constant energy bandgap and the graded energy bandgap materials, respectively. The common techniques of using these materials is described and quantified for both materials. Two models are developed. One model estimates the maximum work rate which can be extracted from a radiation energy flux. This model does not take into account the specific mechanisms of energy conversion inside the work extractor. It provides paramount performance indicators which can be used as a reference for any particular method of work extraction. The second model takes into account specificities of photovoltaic conversion. The performance indicator is the work content factor, which is denoted κtot,max and κel,max for the first and second model, respectively. Solar energy conversion into work is treated as an application. Photovoltaic conversion based on materials with constant energy bandgap is able to extract electric work which is up to a half of the maximum work that can be generated. When materials with graded energy bandgap are used, in the ideal case of transparent atmospheres and null reflectances, κtot,max is around 0.93 while κel,max ranges between 0.73 and 0.86, depending on the material temperature and the radiation concentration ratio. In the more realistic case of semi-transparent atmospheres and materials provided with antireflection layers the two indicators range around 0.87 (for κtot,max) and between about 0.78 and 0.85 (for κel,max). Therefore, photovoltaic conversion based on materials with graded energy bandgap is able to extract a large part (but not all) of the work that can be generated.